1. Field of the Invention
This invention relates to a photopolymerizable composition comprising a chlorinated polyolefin as polymeric binder, a monomer having one unsaturated olefinic bond capable of addition polymerization and an initiator promoting the photopolymerization of the monomer. This invention also relates to recording materials prepared by coating on a plastic film support a layer of the photopolymerizable composition and further, if desired, providing a protective plastic film over this layer.
2. Description of the Prior Art
Photopolymerizable compositions, utilized in photo resists, offset plate masters, relief printing plates, transfer type photopolymerizable materials and, moreover, recording materials such as "Crolux", trade name, produced by E. I. DuPont de Nemours, which is based on a photoadhesion phenomenon, usually comprise (a) a polymeric material as a binder, (b) a monomer having an ethylenically unsaturated linkage, (c) an initiator for photopolymerization, and other ingredients. Some specific compositions are disclosed, for example, in the following patent specifications; Japanese Patent Publication No. 16614/1960, Japanese Patent Publication No. 15810/1963, and Japanese Patent Publication No. 9663/1963 and U.S. Pat. No. 3,353,955. Here, the monomer having an unsaturated double bond functions, prior to exposure to radiation, as a plasticizer or a tackifier for the polymeric binder used, and upon exposure to actinic radiation forms three-dimensional cross-linkages, thus losing its previous plasticizing or tackifying characteristics. Therefore, how well the monomer can exhibit its intrinsic properties depends to a great extent on the kind, the properties or the mixing ratio of the polymer binder.
Especially, where a photopolymerizable composition is used for the preparation of a printing plate, the composition must exhibit, first of all, a sufficiently high photosensitivity for practical applications. However, from the practical point of view, other characteristics are also quite important including the oleophilic nature of the image area (in other words, the degree of hydrophilicity in the background area), the ease with which the developing operation can be carried out, and a high resolving power. If the resolving power is rather poor, the middle portion of the English letter "o" or "e" will be filled in, which cannot be permitted in the printing master. Considering these aspects, the interaction between the polymer binder and the monomer is very significant.
Further, in the case of a transfer type photopolymerizable recording material, in addition to an acceptable level of polymerization speed, a distinct difference must be developed in the exposed and non-exposed areas in some physical property, for example, such as softening point, adhesive property or hue (color formation or color fading). Such a difference in a physical property results at the subsequent transfer procedure in an imagewise transfer either of the exposed or non-exposed area onto a receiving material. In the transfer procedure, the system may be heated, if necessary.
Recently, a recording process has been developed in which the difference in the surface tackiness of a photopolymer layer at the exposed and non-exposed areas is detected by dusting a finely-divided coloring powder (for example, a toner powder) to provide proofs for multi-color printing. In these applications as in the preparation of the printing plate, the selection and combination of a polymer material as the binder and of the monomer used is quite important in order to increase the difference in properties between the exposed and non-exposed areas.
On the other hand, image forming processes are known as described in detail in some patent specifications including Japanese Patent Publication Nos. 9663/1963 and 7728/1972 and U.S. Pat. No. 3,353,955 wherein an image is obtained by the use of a difference in adhesive properties at the exposed and non-exposed areas. This method, using a recording material prepared by providing a photosensitive layer mainly comprising as components a polymeric binder, an ethylenically unsaturated monomer and a photopolymerization initiator as described above on a support such as a plastic film, a metal plate or a paper, and then placing a thin transparent film over this layer, comprises: an image exposure through an original from above the film, and a peeling-off of the film whereby either the exposed or non-exposed area of the photosensitive layer stays unchanged on the support and the complementary, remaining area is transferred or held on the cover film, thus giving a negative and positive image on each of the support and the cover. In this process, a separation of the photosensitive layer essentially takes place at the boundaries between the exposed and non-exposed areas. The mechanical properties related with such a phenomenon primarily depend on those of the polymer binder and of the monomer used in the layer. The most important factors are the molecular weight, the softening point, the degree of crystallinity, the compatibilities with other materials, and the adhesion strength to the support of the polymer binder used. The resulting image, which will subsequently be utilized for various applications, further should meet certain requirements depending on the end use application, such as resistance to abrasion, chemicals and to weathering, etc.
The importance of the properties of the ingredients used in the recording materials comprising a polymer binder, an ethylenically unsaturated monomer and a polymerization initiator has been explained. However, most materials already known in the art do not at present completely satisfy all the requirements described above, and exhibit a rather poor image forming capability. In Japanese Patent Publication 9663/1963, polymer materials are disclosed as binder which can also impart tackiness to the recording layer. The materials disclosed include polyvinylbutyral, polyvinyl acetate, polyvinylpyrolidone, gelatin, coumarone-indene resins, silicone resins, rubbers, etc. Also, Japanese Patent Publication 7728/1972 discloses that polymers such as vinylidene chloride copolymers, cellulose esters, synthetic rubbers, polyvinyl acetate, polyalkyl acrylates, polyvinylchloride, etc. can be used as a binder. However, according to our experimental evaluations, combinations of these polymers with a variety of monomers and of polymerization initiators failed to give satisfactory results. Especially, with respect to polyvinyl chloride, U.S. Pat. Nos. 3,867,153 (MacLachlan) and 3,261,686 (Celeste et al) disclose that polyvinyl chloride are used in combination with monomer for light sensitive materials. However, from the results of our detailed experiment, as polyvinyl chloride has defects that polyvinyl chloride has poor compatibility with monomer, furthermore, there is a few organic solvents capable of dissolving polyvinyl chloride so that kind of the organic solvent is limited, it is clear that polyvinyl chloride is very unsuitable for the effective use as the binder of photopolymerization composition.